1. Field of the Invention
The present invention relates to a display control device for displaying in gradation a picture which is an output from a computer system, for example, on a monochrome display device such as a monochrome liquid crystal display.
2. Description of the Background Art
FIG. 1 is a schematic block diagram showing this kind of conventional display control device. As shown in FIG. 1, a synchronizing signal generator 1 receives a clock signal CLK to generate horizontal and vertical synchronizing signals. A gradation level generator 2 receives the vertical synchronizing signal 3 of these synchronizing signals to generate a group of gradation level signals 4 shown in FIG. 2. In the case of a display control device for liquid crystal display, for example, the clock signal CLK is referred to as a dot clock signal, and the vertical synchronizing signal 3 as a frame signal, respectively.
Usually, a frame thinning system is employed on carrying out gradation display on a monochrome liquid crystal display. The "frame" herein means a cycle during which one picture is displayed on a display, that is, a duration beginning with displaying the first line of a certain picture on the display through the display of the last line to just before the display of the first line of the next picture. In the case that a dot on the display is displayed at m/n gradation level by frame thinning method, data on the dot is fixed at "H" (e.g., black) for m frames of n frames, and fixed at "L" (e.g., white) for the remaining frames.
The group of gradation level signals shown in FIG. 2 includes 8 gradation level signals for 8-gradation display, 0/3 (0%), 1/3 (33%), 2/3 (66%), 3/3 (100%), 1/5 (20%), 2/5 (40%), 3/5 (60%) and 4/5 (80%). In the gradation level signal of 1/3 (33%), for example, the first frame is "H", the second and third frames are "L", the fourth frame is "H" . . . , and so continued. The "H" level represents display ON (to work) and the "L" level represents display OFF (not to work). Accordingly, as to the 1/3 gradation level signal, a dot on the display can be displayed once per three frames (displayed in black on a monochrome display, for example). Usually, the frame cycle is very short, approximately for 15 m sec. Therefore, when a dot is displayed with the 1/3 gradation level signals, this dot is visible one third in the depth of black compared with the case in which the whole frames are at display ON (3/3) stage. Similarly, display with 2/3 gradation level signals is darker than that with 1/3 gradation level signals and lighter than that with 3/3 gradation level signals.
The group of the gradation level signals 4 is applied to the selecting circuit 5 at the data inputs D0 to D7. The selecting circuit 5 also receives an intensity signal I, a video signal V and a color signal R2 at its selecting inputs SA to SC. The signals I and V are used for displaying a picture on a monochrome CRT, and the signal R2 is one of signals used for displaying a picture on a color CRT. The signals I and V are usually used for gradation display on a monochrome liquid crystal display, and the signal R2 is additionally used for enabling 8-gradation display in the display control device in FIG. 1. The selecting circuit 5 selects any one of the gradation level signal group 4 in accordance with Table 1 below with the signals I, V and R2 to output from the output Y.
TABLE 1 ______________________________________ R2 V I GRADATION LEVEL ______________________________________ 0 0 0 0/3 (0%) 0 0 1 1/3 (33%) 0 1 0 2/3 (66%) 0 1 1 3/3 (100%) 1 0 0 1/5 (20%) 1 0 1 2/5 (40%) 1 1 0 3/5 (60%) 1 1 1 4/5 (80%) ______________________________________
As shown in Table 1, any one of the gradation level signals 0/3 to 3/3 is selected in response to the content of 2 bit of the signals I and V when the signal R2 is equal to 0, whereas any one of the gradation level signals 1/5 to 4/5 is selected in response to the content of 2 bit of the signals I and V when the signal R2 is equal to 1. The gradation level signal selected in this way is applied to a monochrome display device such as a monochrome liquid crystal display, whereby the previously mentioned gradation display by means of frame thinning can be carried out.
The conventional display control device constructed as described above switches a gradation level mainly in response to 2 bit signals I and V for a monochrome CRT. For a color CRT, on the other hand, color signals R1, G1 and B1 corresponding to the primary colors of light, i.e., red, green and blue as well as color signals R2, G2 and B2 for fine adjustment therefor are often used instead of the signals I and V.
When the computer system connected to the display control device shown in FIG. 1 operates with software for monochrome display, a picture outputted from the computer system is displayed with the signals I and V, but when it operates with software for color display, the picture is displayed with the signals R1 to B2. Consequently, when software for color display is used for the computer system connected to the display control device shown in FIG. 1 in order to display with gradation the picture outputted from the computer system on a monochrome liquid crystal display by the output from the selecting circuit 5, the gradation level hardly changes because the signals I and V does not change, so that the resulting picture becomes inconspicuous.